1. Field of the Invention
This invention is directed to the production of wire insulation varnish resins which can be readily employed in the melted form. More particularly, this invention is directed to the use of wire insulation varnish resins which can be employed without the use of solvents in the melted state and which have a desirable viscosity characteristic such that high temperatures are not required to obtain the resin in a suitable workable form. This invention is directed to the process of making such resins.
2. Discussion of the Prior Art
It has been known to prepare wire insulation varnish resins from terephthalic and/or isophthalic acids and their esters using a more than bivalent alcohol and a diol. After these materials are dissolved in a cresol-containing solvent mixture containing a hardener such as alkyl titanate, they are applied to a wire conductor and baked thereon.
It is furthermore known to apply such resins in the melted state, without the use of solvents (Yu. I. Linin, Tr. Tomsk. Nauch., Issled. Inst. Kabel. Prom. 1969, No. 1, 231-39). This procedure has considerable advantages over application from solution, one prominent advantage being the fact that pollution of the air due to cresolic solvents which evaporate during the baking process is entirely eliminated.
Resins of the above-described composition, however, provide the conductors with coatings which no longer fully satisfy today's requirements. It has therefore been proposed to replace the radicals of terephthalic acid and/or isophthalic acid wholly or partially with those of a dicarboxylic acid having two imide groups and having the general formula ##STR1## wherein --N--R--N-- represents the radical of a diprimary diamine, preferably of 4,4'-diaminodiphenylmethane.
This dicarboxylic acid containing two imide groups may also be produced in situ in the presence of all or part of the other resin-forming starting materials in the course of the resinification, from trimellitic acid anhydride and diamine in a molar ratio of about 2 to 1, and may be condensed with those materials. This procedure is disclosed, for example, in German "Offenlegungsschriften" Nos. 1,520,061 and 1,495,456.
Another known device is the use of tris-(hydroxyethyl)-isocyanurate as a polyfunctional alcohol in the resins of the initially described composition (cf. German "Offenlegungsschrift" No. 1,590,442).
The resins corresponding to the present-day state of the art accordingly contain radicals of the above-mentioned dicarboxylic acid containing two imide groups, in addition, in some cases, to radicals of terephthalic acid, and also radicals of tris-(hydroxyethyl)-isocyanurate and ethylene glucol. Such products are marketed in the dissolved state with hardeners added as wire insulating varnishes, and to a great extent they satisfy the requirements made of such varnishes (cf. German "Offenlegungsschrift" No. 1,645,435, for example).
Resins of such composition have comparatively high softening points which softening points increase as the degree of condensation increases. They are therefore scarcely suitable for application from the melt, since at relatively low temperatures of 140.degree. to 160.degree. C. they have too high a viscosity and at higher melting temperatures -- say of 160.degree. to 180.degree. C. -- their viscosity is changed by further condensation in proportion to the detention time. To remedy this difficulty it has been proposed that comparatively small amounts of a solvent be added to such resins, or that they be made in the presence of a solvent, in such a manner that a sufficiently low melt viscosity will be obtained at a temperature that will prevent further condensation (German "Offenlegungsschrift" No. 2,135,157).
This procedure, however, is not very advantageous for two reasons: (1) On the one hand, the initially mentioned problem of atmospheric pollution is not eliminated but only diminished, and (2) on the other hand, the viscosity of the melted resins containing solvent changes in the course of time due to the evaporation of the solvent at the melting temperature, at least when the melt is in open containers such as those commonly used with the wire varnishing machines of the conventional type.
It has therefore become desirable to provide a wire insulation varnish resin which can be applied in the melt and which does not require the use of cresol-containing solvents. Moreover, it has become desirable to provide a varnish resin whose viscosity is suitable to allow the resin to be applied to a conductive substrate employing temperatures of 140.degree. to 160.degree. C.